Ac regulator employing a control rectifier switching circuit

ABSTRACT

An AC regulator which employs inductance, a resonant circuit and controlled rectifier switching between the input terminals connected to the AC voltage to be regulated, and the output terminals which supply the regulated AC output. The controlled rectifier switching circuit is regulated by a control system which monitors the output voltage and controls the firing of a pair of oppositely-polarized parallel controlled rectifiers connected in serial relationship between the input (line) and output terminals.

United States Patent Inventor George A. Gautherin Woodside, N .Y.

Appl. N o. 7 62,934

Filed Sept. 26, 1968 Patented Aug. 24, 1971 Assignee Lambda ElectronicsCorporation Huntington, N.Y.

AC REGULATOR EMPLOYING A CONTROL RECTIFIER SWITCHING CIRCUIT 3,263,1577/1966 Klein 323/22 (SCR) 3,303,414 2/1967 Jensen.... ...323/22 X (SCR)3,342,167 9/1967 Tarter 307/252 X 3,370,223 2/1968 Senetcen.... 323/45 X3,384,763 5/1968 Harris,'.lr.... 307/252 3,448,371 6/1969 Boymel 323/24X 3,470,448 9/1969 Krabbe 321/9 Primary Examiner-J. D. Miller AssistantExaminer-A. D. Pellinen AttorneyMorgan, Finnegan, Durham and Pine Lin T53 I Ia 1 -----T I a l {g u l a 5 l w, 5 w 08, l V Z 4 V0 K2 2 l 1 vPATENTED AUG24 i971 SHEET 1 OF 2 LN'VEN'I'OR. GEORGE GAUTHER/N DUN"! MANull hrxu ,MMMQL ATTORNEYS PATENTED AUB24|97I 3.601; 686

sum 2 UF 2 A VL mm. h lo INVEN'I'OR. GEORGE GAUTHERIN A TI'URNE Y5 ACREGULATOR EMPLOYING A CONTROL RECTIFIER SWITCHING CIRCUIT This inventionrelates to AC regulators employing a resonant circuit together withswitching means for controlling the duty cycle and vector relationshipsin the regulator to thereby maintain a regulated AC output voltagenotwithstanding changes in the input (line) voltage, changes in load andchanges in line frequency.

It is one object of the invention to provide an AC regulator whichprovides improved performance while at the same time substantiallyreducing the complexity and number of circuit elements necessary toeffect regulation.

A further object of the invention is to provide a regulator having animproved range of operation and particularly one in which regulation ismaintained even in the presence of unusually large increases in linevoltage or changes in line frequency.

A still further object of the invention is to provide an improved ACregulator in which, under conditions of minimum line voltage and maximumoutput voltage, the maximum current capability of the regulator issubstantially the same for both resistive and inductive loads.

A still further object of the invention is to provide improved controlmeans for an AC regulator including an improved output monitoring anderror circuit which compensates for wave form distortion without theneed for conventional r.m.s. detectors. I

An additional object of the invention is to provide an AC regulatorhaving an improved control circuit in which auxiliary power and a sourceof trigger pulse for the controlled rectifiers arederived from the samecircuit configuration.

Other objects and advantages of the-invention will be set forth in parthereinafter and in part will be obvious herefrom, or may be learned bypractice with the invention.

Serving to illustrate an exemplary embodiment of the invention are thedrawings of which:

FIG. 1 is a circuit diagram of an illustrative embodiment of theinvention; and

FIGS. 2A, 2B and 2C are vector diagrams illustrating deficiencies incertain prior art circuits.

The general organization of the illustrative embodiment is shown in FIG.1, the heavy lines indicating the power flow circuit while the thinnerlines depict the control circuit as sociated therewith.

The alternating current source, the voltage V, of which is to beregulated, is connected to terminals I, and I, of FIG. 1. In the lineconnected to terminal I, a circuit is provided consisting of twobranches connected in parallel. The first branch comprises the seriescombination of choke L,,, and control rectifier CR,; the second branchcomprises the series combination of choke L and controlled rectifier CRThe control rectifiers are oppositely polarized relative to terminal I,.

From this circuit, the line connected to terminal I,, is connected to atap on inductor L Connected in parallel with L, are three branches, thefirst branch consisting of capacitor C,,, a second branch consisting ofinductor L and capacitor C series relationship, and a third branchconsisting of the series combination of inductor L, and capacitor C,.

The combination of L and the above described branches connected inparallel therewith, forms a circuit which is tuned to resonate at thefrequency of the input line voltage V,,. This frequency will typicallybe that characteristic of various com mercial AC sources, e.g. 60 hertzor 400 hertz. In addition, the three branches L, C,, L, C, and C, formfilters for the third, fifth and higher frequency components of thesignal applied to the resonant circuit.

One common junction of the resonant circuit is connected to inputterminal I, and to output terminal 1,. A tap on the inductor L is alsoconnected to output terminal I, via a resistor R The regulated voltageV, appears across output terminals I 1,. The circuit illustrativelyutilizes an autotransformer connection to obtain the desired outputvoltage range. In some cases an isolation transformer can be used forthis purpose.

The control system for controlling the regulating action includes adiodebridgeDB connected via sensing leads to the output terminals I, andI,. The rectified voltage appearing at the output terminals of the diodebridge is applied to the error detecting circuit to be described morefully hereinafter. Generally speaking the error detecting circuitdevelops an error signal in PNP stage 0,. The error voltage is afunction of variations of the output voltage from the prescribed value.

The amplified error voltage'appearing at the collector of Q, is appliedto a further stage which includes an NPN transistor Q The output of Qappearing at the emitter thereof is applied to a diode bridge DB, whichis connected across secondary winding W, of a transformer T The primaryof T comprises a winding W which is energized by a current derived fromthe input terminals 1,, 1,.

The circuit supplying this current to W comprises a series combinationof capacitor C,,,, capacitor C,, and W That combination is connectedacross I, and I, and is therefore energized by-V,,.

The alternating current flowing in this series circuit causes a periodicvoltage rise across capacitor C,, and that voltage, denoted V is used totrigger the control rectifiers CR, and CR2.

To accomplish this there is connected across C,, a series combination ofa bilateral switch S a resistor R and an input winding W,, of atransformer T,.

When the voltage across C,, reaches a predetermined amplitude thebilateral switch 8,, fires and is polarized to conduct current in onedirection or the other depending upon the polarity of V When switch Sconducts, current flows in winding W, causing a trigger voltage to beinduced in the two secondary windings W, and W Since switch S fires onceduring each half cycle of input voltage V, a corresponding voltage pulseis induced in the windings W, and W, during each half cycle. Thesevoltages are applied to the respective gate circuits of the controlledrectifiers CR, and CR Both rectifiers accordingly conduct when theirgate circuits receive the correct polarity voltage from their respectivewindings W, and W,. The actual conduction time determined by the controlaction is described more fully hereinafter. When the rectifiers conduct,current may flow between the input and output terminals of theregulators, the duty cycle of this current being controlled by thecontrol system. Through the regulation of this duty cycle in conjunctionwith the phase shift action of the circuit, the output voltage V, ismaintained constant notwithstanding changes in the input (line) voltageor load.

The duty cycle control is influenced by the conduction of O, which actslike an adjustable load on secondary winding W With 0 saturated, maximumphase shift is realized. In this condition, W is effectivelyshort-circuited and the circuit containing W, acts capacitive causing Vto be in phase with V With 0,, off, the circuit containing W act sinductive and V now lags V, by Accordingly, the phase relationshipbetween V, and V, can vary over a range of 180 to provide a wide controlof the firing angle of the controlled rectifiers.

.The basic regulating action generally described above may be furtherunderstood by reference to the vector diagrams of FIG. 2. In FIG. 2A,the first condition is depicted by the vectors V V 1, and V,,. Thelatter represents the drop across the choke circuit of FIG. I. For easeof explanation the vector diagram excludes the voltage step-up effectedby the autotransformer connections to the resonant circuit.

The second condition is also illustrated in FIG. 2A, in this case by thevectors V,, V,,', I and V,,'. Note that while the output voltage vectoris shifted angularly it has remained substantially constantnotwithstanding the increase in line voltage as exemplified by thedifference in magnitude between the original line voltage V, and the newline voltage V,

The regulation technique involves the power relationship In L L o o o oL where 1 is power input (real) PF is power factor, and I is poweroutput Certain simplifications have been adopted. Losses are lumped inthe P term and input current I, is assumed equal to output current l Forconstant P and V 1,, and l, are constant. Therefore P or the product V lPF must be constant. If V,, changes then PF must inversely change tomaintain the desired constant condition.

FIG. 2A illustrates the change in PF as reflected in the angular shiftof 1,, and V relative to V, Note also that V appears to have varied asPF varied; The voltage V (considering just one branch LK,, CR is the sumof the voltages V,,, across Ll(,, and V, across CR,. Since V varies withPF but I is to remain constant, then the voltage V,, across LK, mustremain constant. Therefore the voltage V must increase to account forthe required increase in V This action is accomplished by the duty cyclecontrol. An extension of the above will show the regulation effect inthe presence of changes in load or line frequency.

One of the primary functions of the control circuit is to regulate thefiring angle of the controlled rectifiers thereby controlling the dutycycle such that the output r.m.s. voltage remains substantially constantwhile at the same time the voltage V increases due to the increased dropV across the controlled rectifier. Accordingly, an increase in linevoltage is accompanied by a shorter conduction period of the controlledrectifier circuit and choke circuit so that thevoltage developed in theresonant circuit including inductor L remains the same as it was priorto the increase in line voltage.

The vector diagrams of FIGS. 2B and 2C illustrate the circuit actioncharacteristic of a number of prior art AC regulators.

FIG. 2B depicts the condition obtaining when linevoltage V is at itsminimum value and V is at its maximum value and with a resistive load.Under these conditions, for an assumed minimum resistive load an outputcurrent 1,, will flow as represented in FIG. 23. That value of 1produces the choke voltage V of F i6. 28.

FIG. 2C depicts the condition existing when V is again at its minimum,V, is again at maximum and an inductive load is connected across theoutput terminals. The resultant output current under these conditions isdepicted as 1,, in FIG. 2C.

These vector diagrams illustrate that the current 1,, for an inductiveload under the conditions where V is minimum and V, is maximum issubstantially less than the available current 1,, or l, under the sameline voltage and output voltage conditions for a resistive load.

To overcome this deficiency the circuit of the present invention employsin its illustrative embodiment two series chokes L and L each seriallyassociated with a respective one of the control rectifiers CR and CRUnder conditions of a reactive load where as noted above the outputvoltage would tend to drop, the control system so shifts the firingangles of the controlled rectifiers that their conduction periodsoverlap. This has the effect of reducing the effective reactance of thechoke circuit because of the resultant shift in the operating point onthe BH curves of the choke. This overlapping conduction also has theeffect of adding increased r.m.s. voltage to the chokes by way ofincreasing their effective volt-time integrals. As a consequenceadditional output current is made available.

' under low line voltage and inductive load conditions The outputvoltage V, will in practical situations include some waveformdistortion. This complicates theproblem of obtaining a sampling signalwhich is indicative of the r.m.s. value of the output voltage.

In accordance with the invention this problem is alleviated by a simpletechnique which involves the development of a sample of the outputvoltage which is a function of both the v peak and average values. Aproper weighting factor applied to those values yields very nearly thecorrect r.m.s. value of the wave where it is periodic and continuous.

resistor R NPN transistor Q and adjustable potentiometer:

P The base of Q is connected to the output end of P via a resistor R Thetransistor Q, functions as a constant current source to provide areference current in the branch which is responsive to both the peakvalue of the output voltage and its average value. The voltage at thecollector of Q will accordingly reflect changes in these parameters ofthe output voltage and will therefor respond to the r.m.s. value. Thatcollector voltage is connected in turn to the base of the errorconducting transistor stage Q Applied to the emitter circuit of Q: is areference voltage across the zener diode D That diode is connectedthrough the emitter to semiconductor diode D The zener D,, is energizedin turn through the output of the auxiliary supply bridge controllingthe firing times of the controlled rectifiers CR and CR by controllingthe phasing of the voltage V, developed across C as previouslyexplained. Control is such that a full range is obtainable.

The circuit of FIG. 1 also includes a current limiting function. Outputcurrent I, fiows through resistor-R controlling the voltage drop acrossthat resistor. That voltage is supplied in turn to a winding W5 of atransformer T The voltage induced in secondary winding W. of Taccordingly. depends upon the magnitude of output current. ,That voltageis rectified by a diode bridge DB and is then applied across a filter Cand a potentiometer P connected in parallel with the filter. Anadjustable voltage may be derived from the arm of P and is applied viazener diode D to the base of 0,. When output current exceeds thepredetermined limit as set by P the voltage from P is sufficient toalter the conduction of 0,, thereby maintaining the firing of controlrectifiers CR and CR to the angles which satisfy the current limit.

It should be understood that in studying and practicing the invention,modifications will undoubtedly occur to those skilled in the art. Theinvention is thus not to be limited to the specific circuits andcomponents herein shown and described, but departure may be madetherefrom within the scope of the accompanying claims.

1. An AC regulator for generating a regulated AC output voltageindependent of changes in line frequency, line voltage or load, said ACregulator comprising a. input and output terminals;

b. a line resonant circuit tuned to resonate at the frequency of theinput line voltage, said line resonant circuit being connected betweensaid input and output terminals;

c. parallel choke means connected in serial relationship between saidinput and said line resonant circuit;

d. switching means connected in serial relationship with each of saidchoke means for regulating the flow of current between said input andoutput terminals, said switching means comprising oppositely poledcontrolled rectifiers;

e. control means responsive to the peak and average values of the outputvoltage appearing at said output terminals and operatively coupled tosaid switching means for controlling the firing times of said switchingmeans, said control means comprising:

i. a bridge rectifier having input and output terminals, said inputterminals being operatively coupled to said output terminals of theregulator;

ii. an RC network operatively coupled to said output tercoupledcapacitance of said primary circuit;

minals of said bridge rectifier, the resistance and vi. a bilateralswitch operatively coupled to said serially capacitance of said networkbeing serially coupled and coupled capacitance responsive to a minimumvoltage proportioned to g nerat 3 ign l in po 10 the appearing acrosssaid serially coupled capacitance,said P and average Values QfsaidOutput Voltage; 5 switch being capable of conducting current in one ofiii. a detector circuit for converting said peak and average twodirections dependent upon the characteristics f signal values to asignal indicative of the r,m.s. value of the output independent of waveshape; iv. means for applying said signal to a solid-state device forregulating the conductivity of said device; 10 v. a transformer having aprimary circuit comprising a primary winding with serially coupledcapacitance and a secondary circuit comprising a secondary winding, saidprimary circuit being operatively connected across said input terminalswhile said secondary winding is operal5 tively connected across theoutput of said solid-state the voltage appearing across the capacitanceserially coupled with said primary winding;

vii. means responsive to the flow of current passed by said bilateralswitch for generating a voltage capable of ac tuating said switchingmeans such that a regulated output voltage appears at said outputterminals.

2. An AC regulator as described in claim 1 additionally comprisingcurrent limiting means operatively coupled between said line resonantcircuit and said output terminals,

device, the conductivity of said solid-state device being said w f imeans blamg .capable of generating an determinative of the impfidancecharacteristics of Said error signal indicative of variations in outputcurrent for conprimary circuit so as to vary the phase relationshiptrolling the firing times of said switching means so as to attainbetween the voltage appearing across said input teraregulated outputminals and the voltage appearing across the serially

1. An AC regulator for generating a regulated AC output voltageindependent of changes in line frequency, line voltage or load, said ACregulator comprising a. input and output terminals; b. a line resonantcircuit tuned to resonate at the frequency of the input line voltage,said line resonant circuit being connected between said input and outputterminals; c. parallel choke means connected in serial relationshipbetween said input and said line resonant circuit; d. switching meansconnected in serial relationship with each of said choke means forregulating the flow of current between said input and output terminals,said switching means comprising oppositely poled controlled rectifiers;e. control means responsive to the peak and average values of the outputvoltage appearing at said output terminals and operatively coupled tosaid switching means for controlling the firing times of said switchingmeans, said control means comprising: i. a bridge rectifier having inputand output terminals, said input terminals being operatively coupled tosaid output terminals of the regulator; ii. an RC network operativelycoupled to said output terminals of said bridge rectifier, theresistance and capacitance of said network being serially coupled andproportioned to generate a signal in response to the peak and averagevalues of said ouTput voltage; iii. a detector circuit for convertingsaid peak and average signal values to a signal indicative of the r.m.s.value of the output independent of wave shape; iv. means for applyingsaid signal to a solid-state device for regulating the conductivity ofsaid device; v. a transformer having a primary circuit comprising aprimary winding with serially coupled capacitance and a secondarycircuit comprising a secondary winding, said primary circuit beingoperatively connected across said input terminals while said secondarywinding is operatively connected across the output of said solid-statedevice, the conductivity of said solid-state device being determinativeof the impedance characteristics of said primary circuit so as to varythe phase relationship between the voltage appearing across said inputterminals and the voltage appearing across the serially coupledcapacitance of said primary circuit; vi. a bilateral switch operativelycoupled to said serially coupled capacitance responsive to a minimumvoltage appearing across said serially coupled capacitance, said switchbeing capable of conducting current in one of two directions dependentupon the characteristics of the voltage appearing across the capacitanceserially coupled with said primary winding; vii. means responsive to theflow of current passed by said bilateral switch for generating a voltagecapable of actuating said switching means such that a regulated outputvoltage appears at said output terminals.
 2. An AC regulator asdescribed in claim 1 additionally comprising current limiting meansoperatively coupled between said line resonant circuit and said outputterminals, said current limiting means being capable of generating anerror signal indicative of variations in output current for controllingthe firing times of said switching means so as to attain a regulatedoutput.